 This schematic diagram is a much more up-to-date version of what we think is going on in the interior of the Earth, at least in the mantle. And the take-home message here is motion. See all these little black arrows everywhere? They are showing you that the mantle is not actually just statically sitting there. It's moving around all the time. And the thing that drives that motion is internal heat. So the core has a lot of excess heat from the formation of the Earth and from the decay of radioactive elements. And it needs to get rid of that heat somehow. And the way it does it is by convection. And that means moving hot material from one place to another where it can give that heat away. So from the core-metal boundary up, first of all, you've got this kind of weird D-double prime layer where strange things happen to seismic waves that get in there. And here is a plume of material that's buoyantly rising because it's hot. And this is positive to have been the source for hotspot volcanoes. This one pictured here. Now we also have arrows that show things that are sinking. So right here is a cross-section of a subduction zone. And you can see the slab is sinking down here. A lot of slabs get sort of hung up at around 670 kilometers depth. This is where the mantle has an increase in density. And so it's harder for a sinking slab to get through there, but they do get through most of the time. And when they do, the material that composes them piles up down here, so it can later be recycled into whatever the rest of the metal is doing, right? So the take-home message here again is motion. But I want you to also remember that we're talking about solid rock here. This is by no means a liquid. So that motion is happening on very long time scales.